This is an interesting question, since there are many potential stock characters in Of Mice and Men, by John Steinbeck. Because it is a novella (short novel), none of the characters in this work are fully developed; there is just not enough time for that in such a short work. The purpose of this literary format is often to focus on the story rather than the characters.
A stock character is "a fictional character based on a common literary or social stereotype. Stock characters rely heavily on cultural types or names for their personality, manner of speech, and other characteristics." For example, if one says "she was a typical cheerleader," we would probably go to the stereotype and assume she is blond and rather ditzy; her boyfriend, the "typical football player," is of course huge and good-looking but not very bright. Given that definition, nearly every one of the characters in this novel is a stock character.
Lennie is the typical huge but not-very-bright man who is harmless but gets himself in trouble because of his size or mental capacity. We have many recognizable "Lenny" characters, including Foghorn Leghorn in the old cartoons. He seems imposing or intimidating because of his size but is generally a danger to no one but himself.
George is another stereotype: the small-but-wiry sidekick (or nemesis) of the gentle giant, who makes all the decisions because the big guy cannot. He is also a typical dreamer, always talking about things he wants but never really getting there--and we all, including him, know his dreams will never be realized.
The rest of the characters are nearly all stereotypes or stock characters:
Slim, the knowledgeable, exceptionally capable, stoic ranch hand; Curley, the ranch owner’s diminutive son, who is intensely jealous of his wife and quick to pick fights to prove his prowess; and Curley’s wife, a flirtatious young woman convinced that she was destined for a more glamorous life than the one she leads on the isolated ranch.
While most readers would not consider George and Lennie as stock characters (though it is reasonable to do so, based on their appearances and their characterizations), the rest certainly would be. They are all undeveloped characters of a certain type, which is a reasonable definition of a stock character.